In an optical disc drive, laser light is applied to an optical disc and a reflected light is converted into an electric signal to be subjected to signal processing, whereby it is possible to reconstruct a physical digital signal that is formed on the optical disc as an electric signal. In recent years, as the optical discs that utilize this principle, read-only disks and rewriteable disks have been put to practical use. In addition, there are various formats depending on recording densities. Accordingly, in optical disc drives, it is necessary that the type of optical disc medium be judged before the signal reconstruction process. Generally, since the reflected light amount when a prescribed amount of light power is applied to the disk is different depending on the type of the optical disc medium, the level of the reflected light amount is detected in the early stage of the medium judgment to estimate the type of the medium, and then after the servo control is performed, recorded data are read to determine the medium. It is possible to change the estimated medium and read the data again to determine the medium even when the estimation based on the reflected light amount is wrong, but because there area variety of formats, a longer time will be required to start the data reproduction when the medium is erroneously determined in the early medium estimation.
A laser power control circuit that is provided in these optical disc drives performs a control for reducing variations of the laser power which is applied from a laser and keeping the laser power at a constant level even when the operation environments would change.
Further, the operating life of the semiconductor laser would be shortened when a laser power that is higher than a predetermined level is to be obtained. Accordingly, the control of the laser power is important also from the viewpoint of the long-term operation of optical disc equipment.
FIG. 8 is a diagram illustrating a specific circuit that embodies this function. Hereinafter, problems of this conventional circuit will be described.
In FIG. 8, reference numeral 1 denotes a positive power supply terminal, numeral 2 denotes a negative power supply terminal, numeral 3 denotes a photodetector element, numeral 4 denotes a semiconductor laser, numeral 5 denotes a semiconductor laser driving transistor, and numeral 6 denotes a photoelectric converting variable resistor. The components 1˜6 are referred to as an optical pickup unit (OPU), which is denoted by numeral 10. Reference numeral 20 denotes a differential amplifier, numerals 21 and 22 denote input terminals of the differential amplifier 20, respectively, numeral 23 denotes an output terminal of a laser power control circuit, and numeral 30 denotes a reference voltage source that supplies a voltage value Vr. Numeral 100 denotes a laser power control circuit including these components 20˜30, which is usually formed as a semiconductor integrated circuit.
Next, the operation of the conventional laser power control circuit that is constructed as described above will be described. When a current from the positive power supply terminal 1 is supplied to the semiconductor laser 4 through the semiconductor laser driving transistor 5, light emission occurs. A part of the emitted semiconductor laser light is applied to the photodetector element 3, photoelectric conversion is performed with a photovoltaic current, and then the current passes through the photoelectric converting variable resistor 6, resulting in a voltage signal. Hereinafter, this voltage is referred to as a monitor voltage Vm.
The reference voltage source 30 is connected to the negative terminal 21 of the differential amplifier 20, and the above-mentioned voltage signal that has subjected to the photoelectric conversion is inputted to the positive terminal 22. Further, the output terminal 23 of the differential amplifier 20 is connected to the base terminal of the semiconductor laser driving transistor 5. Here, when the terminal voltage of the non-inverting terminal 22 is higher than the terminal voltage of the inverting terminal 21, the voltage of the output terminal 23 increases because the terminal 22 is a non-inverting terminal of the differential amplifier 20, whereby a base-to-emitter voltage of the semiconductor laser driving transistor 5 decreases. Consequently, the current passing through the semiconductor laser driving transistor 5 decreases, the current passing through the semiconductor laser 4 decreases, and the irradiated light power also decreases. Further, since the photovoltaic current of the photodetector element 3 decreases, the terminal voltage of the non-inverting terminal 22 decreases. Conversely, when the terminal voltage of the non-inverting terminal 22 is lower than the terminal voltage of the inverting terminal 21, the laser power control circuit 100 operates in a direction of increasing the terminal voltage of the non-inverting terminal 22 while the current is passing through a loop.
As described above, the connection between the laser power control circuit 100 and the OPU 10 forms a negative feedback loop, and finally the inverting terminal 21 and the non-inverting terminal 22 would have approximately the same voltage.
On the other hand, the luminous efficiency of the semiconductor laser 4 varies greatly, and this means that the levels of the obtained laser power are different even when the same current is supplied. The photoelectric converting variable resistor 6 is for adjusting these variations of the luminous efficiency. The variable resistor 6 makes an adjustment while measuring the laser power from the semiconductor laser 4 so that the voltage of the photoelectric converting variable resistor 6 has a fixed value when a prescribed laser power is obtained. The voltage which is to be adjusted here is the voltage value Vr of the reference voltage source 30 in the laser power control circuit 100.
The OPU 10 that has been adjusted as described above is connected to the laser power control circuit 100 to form a negative feedback loop, whereby the terminal voltage of the photoelectric converting variable resistor 6 is made equal to the voltage value Vr at the power adjustment, and thus the light power that is applied from the semiconductor laser 4 can be controlled to be a constant value.
In recent years, since the breakdown voltage of the transistor becomes lower as the processes of the semiconductor integrated circuit become finer, about 3V of the supply voltage is employed. On the other hand, in order to obtain a high laser power using the semiconductor laser 4, the power supply voltage of the OPU 10 is usually set at about 5V because the forward voltage becomes higher and accordingly it becomes difficult to operate the circuit using 3V of the voltage of the positive power supply terminal 1. The base voltage of the semiconductor laser driving transistor 5 is a voltage which is lowered than 5V by the base-to-emitter voltage (≈0.7V) of the semiconductor laser driving transistor 5. When the connection as shown in FIG. 8 is made under such situation, the terminal voltage of the output terminal 23 of the laser power control circuit 100 will exceed the process breakdown voltage.
FIG. 9 is a diagram illustrating an example of a circuit for connecting the laser power control circuit 100 and the OPU 10 when the voltage value of the positive power supply terminal 1 of the OPU 10 and the power supply voltage of the differential amplifier 20 are different from each other. In FIG. 9, reference numeral 8 denotes a transistor that is not included in the semiconductor integrate circuit. The breakdown voltage of the transistor 8 is sufficiently higher than the voltage of the positive power supply terminal 1. Reference numerals 7 and 9 denote resistors, which function as inverting amplifiers. The ratio between end voltages of the resistors 7 and 9 is equal to the ratio between these resistances. In this case, the base voltage of the transistor 8 is obtained by adding a voltage that is dropped at the resistor 9 and the base-to-emitter voltage of the transistor 8 (≈0.7V). Therefore, when the resistance ratio between the resistors 7 and 9 is appropriately selected, the negative feedback loop can be formed without the terminal voltage of the output terminal 23 of the laser power control circuit 100 exceeding the process breakdown voltage. Refer to Japanese Published Patent Application No. Hei. 2-159780 (FIG. 5).
When an ideal differential amplifier is used in the above-mentioned Prior Art, the terminal voltage of the inverting terminal 21 and the terminal voltage of the non-inverting terminal 22 become equal to each other, whereby the laser power applied from the semiconductor laser 4 becomes constant. However, in reality, a voltage that is referred to as an offset voltage occurs in the differential amplifier 20.
FIG. 10 is a diagram equivalently showing a state where an offset voltage occurs in the differential amplifier. When the offset voltage Vofs occurs, a potential is generated between two terminals of the differential amplifier. Consequently, a potential is generated between the voltage value Vr of the reference voltage source 30 and the monitor voltage Vm, whereby the laser power of the semiconductor laser 4 is not kept constant. The offset voltage is caused by a mismatch between transistors requiring relative accuracy, such as differential transistors that are used at the input of the differential amplifier 20. This mismatch occurs remarkably in MOS transistors, the magnitude of which is inversely proportional to the square root of the gate width×the gate length of the MOS transistor. Therefore, as common measures, the sizes of these transistors are increased or the reference voltage value Vr is finely adjusted to correct the offset voltage.
Because the laser power control circuit is formed as a semiconductor integrated circuit, when the transistor size is increased, the chip size is accordingly increased. Further, as the fine adjustment of the reference voltage is performed using a fuse, the production cost is increased.
In addition, as the photodetector element 3 has a diode structure and the current starts passing in the forward direction when an adjust voltage of the photoelectric converting variable resistor 6 is increased, the adjust voltage is usually adjusted at a relatively lower voltage (approximately 100 mV to 200 mV). On the other hand, since the output of the laser power control circuit 100 is decided by the power supply voltage of the OPU 10, a difference occurs between these voltages as a circuit offset voltage. Assuming that the differential voltage between the reference voltage Vr and the output voltage of the laser power control circuit is Voofsn and the gain of the differential amplifier 20 is G, the offset voltage that occurs in the circuit can be expressed by Voofsn/G. The circuit offset voltage can be reduced by increasing the gain G of the differential amplifier 20, while when the gain is extremely increased, the intersection of the gain of the feedback loop becomes higher, resulting in an enlarged noise bandwidth or a lowered stability of the feedback loop. Accordingly, as a common design value, the gain G of the differential amplifier 20 is suppressed at approximately 1000 times. Since approximately 2V of the differential voltage Voofsn occurs, the converted offset voltage to the input part becomes 2 mV in this design. As this value corresponds to 2% of the original reference voltage, this is not always a negligible value. Since this offset voltage cannot be avoided by the transistor size adjustment, trimming of the reference voltage by the fuse is required, which also leads to an increase in the production cost.
Further, in such cases that there is a potential between the supply voltage of the OPU 10 and the laser power control circuit 100 as shown in FIG. 9, the connection between the laser power control circuit 100 and the OPU 10 must be changed, and further the specifications of the laser power control circuit 100 must be decided depending on the specifications of the OPU 10.